Both electromagnets and permanent magnets have been used to manipulate beams of charged particles. In traveling wave tubes for example, magnets have been arranged around the channel through which the beam travels to focus the stream of electrons; that is, to reduce the tendency of the electrons to repel each other and spread out. Various configurations of permanent magnets have been attempted in an effort to increase the focusing effect while minimizing the weight and volume of the resulting device. In conventional traveling wave tubes, permanent magnets are typically arranged in a sequence of alternating magnetization, either parallel to, or anti-parallel to, the direction of the electron flow. The magnets are usually annular in shape and their axes are aligned with the path of the electron beam.
Reference is made herein to the hollow cylindrical flux source (HCFS) principle described by K. Halbach in "Proceedings of the Eighth International Workshop on Rare Earth Cobolt Magnets", Univ. of Dayton, Dayton, Ohio, 1985 (pp. 123-136). A HCFS is essentially a permanent magnet shell that produces an internal magnetic field which is relatively constant in magnitude. The field, which is perpendicular to the axis of the cylinder, possesses a strength which can be greater than the remanence of the magnetic material from which the HCFS is made.
Ideally, the HCFS is an infinitely long annular cylindrical shell with a circular cross section, that produces an intense transverse magnetic field in its interior working space. No magnetic flux extends to the exterior of the ring structure (except at the ends of a finite cylinder).
Recently, this principle has been applied to PPM structures. In devising these magnetic structures there has been continuing concern on how to maximize the strength of the magnetic field without increasing the mass of the magnetic structure. The present invention addresses this problem, and others.